The objective of this project is to determine whether subgroups of gay men might be appropriate candidates for future HIV-1 vaccine efficacy trials. A HIV-1 vaccine efficacy trial would best be conducted in a population with a predicted high incidence of HIV-1 infection that remains high enough during the length of the trial to determine vaccine efficacy, and for whom adequate follow-up can be obtained. Young gay men, especially those from lower socioeconomic groups including blacks and Hispanics, may be such a population. Two successive cohorts of homosexually active men who are 18 to 25 years old living in New York City, at high risk of HIV-1 infection and negative for antibody to HIV-1 will be enrolled. Sources of the study population will be: gay organizations serving black and Hispanic men , the New York City Department of Health clinics for sexually transmitted diseases and gay bars. The cohorts will be characterized at baseline with regard to demographics, sexual behaviors and other factors known to affect the risk of infection or risk behavior. Through regular followup visits at three and six months after entry and every six months thereafter, the incidence of HIV-1 infection will be determined and changes in high-risk sexual behaviors monitored. The use of successive cohorts will allow us to determine whether groups of young gay men who are at high risk of HIV-1 infection can be consistently recruited. It is anticipated that the incidence of HIV-1 infection will decline during follow-up. The second cohort will allow us to determine whether changes in the incidence of HIV-1 infection in the first cohort reflect participation in the study or secular changes in the community-at-large. Follow-up visits will allow us to accurately describe changes in the incidence of HIV-1 infection and associated risk behaviors.